There's this prehistoric skull that emits weird energy and duh, of course it's alien. It actually comes from a previously unknown "Planet 5", which the Time Lords hid in a Time Loop because it was 2 spoopy 5 u. It uses humans to arrange a satanic cult (literally, there are pentagrams and shit everywhere - Mary Whitehouse would've had a heart attack, before promptly complaining about how anti-Jebus it was in her column) with a human sacrifice so that it may return and possess humanoid form once again. This results in the lady that it takes over turning into the cousin of the Axons and the Host: The Fendahl Core, who likes to pose dramatically and also shares Medusa's ability to transform those who look into her eyes. Dayum, and you thought The Brain of Morbius was the king of plagiarism?

At the end, the Doctor channels his inner Brigadier and decides that the best way to deal with the problem is to blow up the house that the ceremony took place in. The Fendahl Core tries to escape by doing even more dramatic poses in a sequence as baffling as the Fish people dance from The Underwater Menace, but to no avail as it dies in the fire. The Doctor and Leela then fuck off without saying goodbye to the supporting cast, asperusual.

Demonstrating the consistency of this utterly wretched awesome fanbase, this story somehow went from being one of the most overrated Fourth Doctor stories to being one of the most criminally underrated Fourth Doctor stories.